While the U.S. Government has not discovered evidence of terrorism, they are reviewing all the names and comparing them to relevant criminal and terrorism databases. Malaysian officials say at least two other passengers are drawing scrutiny, but they did not say why. The FBI is planning to deploy forensic investigators to assist, but a lot of work is underway. And more about the two passengers with the stolen passports, how in the post-9/11 world were they able to get on? Are there stolen passports checked before they board? Reporter: There is. The question it whether they were entered into the database, and did they bother to check. The standards differ from airport to airport. And they were reported stolen two years ago. That's right. Thanks to Pierre. And now to the massive air and sea search in the south China sea. Two large oil slicks, but no sign of the plane. In beijing, the intended destination, distraught families are waiting for answer. And Gloria is there. Reporter: Behind me is the hotel where so many mothers, brothers, families of loved ones are waiting. There is a special assistance team from Malaysia airlines offering support. But officials are being pressed to give answers when they have far from perfect or complete information. In China, heartbreak for hundreds of family, holding on to diminishing hopes. As hours passed with no news. One woman telling China daily, the family of my neighbor was on that plane. And the anger was building against the airline. They ignored us for so long, this man said. Our relatives were on that plane. And her son was on the flight, she spoke to us. If you want to know how it is to lose a son at the age of 50, it's devastating. Reporter: Any shred of evidence resumed at dawn, and at least 42 aircraft are involved. In a tweet, they announced the pc Orion will help. The U.S. Is sending one with two search and rescue helicopters. They are focusing on oil slicks where it was detected before it vanished. A possible clue. Possible indication that the aircraft never came back. Reporter: The black box could hold answers, but until investigators find it, an enormous task. It has left few clues. We were inside the room with families, praying for the best, bracing for the worst. They want some kind of explanation to make sense of the loss. I'll take it up. It's unimaginable. Many. And now to the conflicted and difficult task much finding the cause of the crash, looking for the black boxes. We looked for clues, and learned that it may have turned around just before it disappeared. What could that indicate? Reporter: It could indicate something was wrong on the aircraft. That is just a possibility we are hearing from the Malaysian transport ministry. And we have a lot of conflicting information. It's really about the black boxes. The secrets they hold. And as you heard, the search is underway, and we're hoping we can unlock the mystery by finding the boxes. This is the actual 777 missing. Mu maintenance issues, but disappeared 36 hours ago. The red eye to beijing took off on the six-hour flight at 12:41 in the morning. 40 minutes later at 1:20, it flew out of the radar coverage area. Ten minutes later, the airline reported their position. The last time they were heard from. How much longer they were in the air, or what direction they were headed is unclear. But this morning, officials say they're investigating the possibility that the plane tried an air turn back. A literal turn around back to the airport. We know that the pilot, 53-year-old zaharie Shaw was experienced. He had been with the airline more than 30 years. But so much we don't know. What was happening in the final moments? Those black boxes will tell us. They are presumably at the bottom of the south China sea. Locater beacons pinging for a month or so. This is over 600 feet, they will be able to find it. That's not a terrible depth of thousands of feet or a rough bottom that would make a major problem. Reporter:er Reporter: They are so critical. In air France 447, the boxes were retrieved, providing the answers to why that jet fell out of the sky. Some are asking whether all the data should be beamed back to land rather than stored on the black boxes. This morning, the search and rescue crews are looking for wreckage so they could listen for the locater pings from the all-important recorders. Boeing and the ntsb would like to assist in this investigation. They have assembled teams. They are poised and ready to go. They are waiting for the request from Malaysia to come in and help with the investigation. Thank you. More now on the mystery from colonel Steven in Washington this morning. Good morning to you. Under what circumstances could you imagine the pilots turning a plane around mid-flight? There are all sorts of reasons they would turn around. Perhaps something went wrong on the airplane. Maybe for a maintenance issue. It's not unusual for an airplane to turn around. If there's a question about the safety of the airplane, the airplane will be turned around and sent back to the original destination. My question is this a we think they turned it around or maybe, or definitive clues. I'm skeptical of this. Yet another one of the many unanswered questions surrounding this incident. David Curley raised something in the piece I want to ask about. In this day and age with so much technology at our disposal, where the black boxes not transmitting to land? Rather than going to the bottom of the ocean? It's a legitimate question.

This transcript has been automatically generated and may not be 100% accurate.

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